Discussion and Solution of Ink Gelation and Swell in Printing

In the process of packaging printing, there are sometimes streaks or different shades of color on the printed matter. This failure of inking and ink off rollers is caused by the gelation of the ink due to the swelling of the pigments and fillers in the ink. The gelation of the ink is due to its high yield value, and the ink is too viscous to become cheese-like. From an application point of view, there are too many pigments and fillers in the ink, which causes the ink to gel or water enters the ink during storage and causes partial flocculation.

For a long time, people have taken various measures against swelling and gelation, including constantly stirring the ink in the ink fountain, installing a stirrer in the ink fountain, or putting an iron stick on the ink of the ink fountain to push the ink toward the ink fountain roller. To maintain the fluidity of the ink. In the operation, medium and high viscosity ink or thinner is used to adjust the ink (must be used with the adjustment, otherwise the ink will continue to swell or gel). Some manufacturers replace the ink and print with ink with good fluidity and strong water resistance.

1 Causes the ink gelation and swelling

The gelation problems of packaging printing inks are occasionally encountered in early oily (oxidation drying and penetration drying) and water-based inks. The causes of gelatinization are very complicated, including the process flow of ink manufacturing, such as the first and second addition of driers, and the type and quality of ink components. When the ink composition changes, various problems will also appear.

The reasons for gelation are:

(1) Large ink viscosity

Because the molecular weight of the resin in the ink system is too large, the viscosity of the ink is too high, so it is easy to gel.

(2) Many solid components

There are too many solid components, even resin binders with low viscosity are no exception.

(3) High ink acidity

Research data and experiments on packaging printing inks indicate that the ink component materials pH, amphoteric solvents, resins, an aliphatic hydrocarbon solvent that does not form hydrogen bonds, and neutral pigments are all related to ink gelation. When the acidity and alkalinity of the pigment does not match the acidity and alkalinity of the binder, a variety of resins, pigments, and fillers can be used to adjust the acidity and alkalinity, or select an appropriate solvent to make up for the improvement.

(4) Performance of pigments and fillers

When studying the chemical reaction of the ink, it was found that when its concentration is large to a certain degree, some substances will mutate. At this time, the active molecules in the ink begin to associate to form aggregates of varying particle sizes. Such aggregates are called micelles or micelles, also known as associated colloids. An important feature of micelles is their solubilization, which increases the solubility of substances that were originally insoluble or slightly soluble, leading to swelling, which accelerates the chemical reaction. This is very likely to have a close relationship with the binder, especially the pigment dispersion process.

(5) High desiccant content or improper order of addition

The drier in the ink can also directly affect the viscosity of the ink during storage and the sinking of pigments and additives. Such as high-viscosity alkyd resin and basic pigment zinc oxide in the presence of a drier will grind and swell the ink during storage. This is due to the reaction between free fatty acids and basic pigments during thermal processing. Therefore, in the grinding process of high viscosity ink raw materials, driers such as lead, cobalt and manganese should not be added. Adding 1% benzoic acid can reduce swelling tendency. The drier also accelerates pigment settling during ink storage. Adding a drier before the pigment is grinded will result in poor storage of the ink, easy to swell, gel and sink to the bottom.

(6) Too much solvent in the ink

The salt-based pigments in the ink react with binders with higher acidity or containing free fatty acids to form colloids. Because the binder itself will agglomerate and gel, the ink will absorb water, and the proportion of the pigment is too high. If the binder and the pigment are used improperly, the miscibility of the binder and the additive of the grease is poor, and the high polymer binder of the acid-soluble structure is Pigment absorption also accelerates gelation.

Among the above reasons, (1) (2) is very easy to avoid and rule out for an experienced ink manufacturer or packaging printing operator. (3) ~ (6) is caused by the gelation of the ink due to the excessively high temperature of the dispersion process, the volatilization of the solvent, and the improper application of auxiliary additives in the ink manufacturing process.

2 Measures to eliminate swelling of gelled ink

(1) The ink is too thick or gelled due to the excessive polymerization degree of the binder, it can be diluted by adding aliphatic hydrocarbon, fat, ketone, etc.

(2) Gelatinized ink that reacts into soaps can be added with rosin, which is easily soluble in linseed oil, and uses high acidity substances to solve the gelation problem.

(3) Adding inorganic salt to water-based ink can control gelation and swelling.

After practical demonstration, the author believes that the improved phosphate barium acid coupling agent has the special function of restoring the original performance of the swollen and gelled printing ink in addition to the excellent dispersion performance of the pigment. Adding 3% to 5% of anti-swelling and breakers in the swollen ink can solve the above problems and restore the original structure and performance of the ink. Adding 1.5% to 2.5% anti-swelling and gel breaker directly to the unswollen and gelled ink can avoid the swelling and gelation of the ink. Toluene can also be used to grind and disperse with the product after being diluted into the ink, which can greatly improve the dispersion of the pigment, and can prevent the packaging printing ink from swelling or gelling during production and storage, and avoid swelling during printing. The occurrence of streaks or lightening caused by gelation and precipitation can also greatly increase the degree of branching of the ink and increase the adhesion of the ink film, thereby effectively improving the fluidity of the printing ink.

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